Onset Of Anoxic Conditions Research Articles

Seasonal variation of arsenic and antimony in surface waters of small subarctic lakes impacted by legacy mining pollution near Yellowknife, NT, Canada

The seasonal variation in lake water arsenic (As) and antimony (Sb) concentrations was assessed in four small (<1.5km2) subarctic lakes impacted by As and Sb emissions from legacy mining activities near Yellowknife, Northwest Territories, Canada. Substantial variation in As concentrations were measured over the two-year period of study in all but the deepest lake (maximum depth 6.9m), including a four-fold difference in As in the shallowest lake ([As]: 172–846μgL−1; maximum depth 0.8m). Arsenic concentrations were enriched following ice cover development in the three shallowest lakes (50–110%) through a combination of physical and biogeochemical processes. Early winter increases in As were associated with the exclusion of solutes from the developing ice-cover; and large increases in As were measured once oxygen conditions were depleted to the point of anoxia by mid-winter. The onset of anoxic conditions within the water column was associated with large increases in the concentration of redox sensitive elements in lake waters (As, iron [Fe], and manganese [Mn]), suggesting coupling of As mobility with Fe and Mn cycling. In contrast, there was little difference in Sb concentrations under ice suggesting that Sb mobility was controlled by factors other than Fe and Mn associated redox processes. A survey of 30 lakes in the region during fall (open-water) and late-winter (under-ice) revealed large seasonal differences in surface water As were more common in lakes with a maximum depth <4m. This threshold highlights the importance of winter conditions and links between physical lake properties and biogeochemical processes in the chemical recovery of As-impacted subarctic landscapes. The findings indicate annual remobilization of As from contaminated lake sediments may be inhibiting recovery in small shallow lakes that undergo seasonal transitions in redox state.

Rare Earth Elements in early-diagenetic foraminifer ‘coatings’: Pore-water controls and potential palaeoceanographic applications

Rare Earth Element (REE) distributions in the ocean bear the fingerprints of several key environmental processes, including vertical particle/organic carbon fluxes, water column/pore-water oxygenation and ocean transports. The use of ‘fossil’ REE analyses in the service of palaeoceanography as redox, water transport or nutrient cycling ‘proxies’ has long been a tantalizing possibility. Here we demonstrate the application of a novel laser-ablation microanalysis approach for the rapid and accurate measurement of the REE composition of early diagenetic ‘coatings’ on fossil foraminifera. By applying this new method to a range of core-top and multi-core samples, we show that ‘authigenic’ REE enrichments on planktonic foraminifer surfaces (REEfs) reflect a primary seawater signature that becomes overprinted during sediment burial due to early diagenetic processes that control the flux of REEs to pore-fluids. Thus ‘light’ REEs (LREEs), and eventually ‘middle’ REEs (MREEs) are generally enriched in foraminifer 'coatings' relative to seawater, while Ce-anomalies (Ce/Ce*) recorded in surface sediments are typically more positive than local seawater values and are further ‘eroded’ during burial with the onset of anoxic conditions in the sediment. Similar patterns have previously been observed in pore-fluid measurements. Indeed, we show that Mn and Fe concentrations measured in foraminifer ‘coatings’ track the availability of these elements in pore-water, indicating that they are not associated with a secondary oxide phase. We propose that these elements, along with REEs are instead adsorbed directly from pore-fluids. In contrast, U in authigenic coatings tracks the removal of this element from solution under sub-oxic conditions, supporting the use of U/Ca in foraminifer coatings as a redox proxy. Although our results confirm a significant early diagenetic influence on REEfs, we also demonstrate the potential utility of ‘Ce-enrichment’ relative to expected seawater values as a palaeo-oxygenation proxy. We support this proposal with down-core measurements of U/Ca and Ce-enrichment spanning the last deglaciation in the sub-Antarctic Atlantic, as well as a global array of LGM measurements, which are found to co-vary with parallel estimates of radiocarbon ventilation age, consistent with a widespread drop in oxygen suggesting an increase in the efficiency of the biological pump. Our results suggest that laser-ablation REEfs measurements may hold some promise for palaeoceanographic reconstruction, in particular through a shift in emphasis away from the reconstruction of primary seawater signals to the analysis of diagenetic impacts that are sensitive to changing hydrographic conditions.

Hydrogen Dynamics in Cyanobacteria Dominated Microbial Mats Measured by Novel Combined H2/H2S and H2/O2 Microsensors.

Hydrogen may accumulate to micromolar concentrations in cyanobacterial mat communities from various environments, but the governing factors for this accumulation are poorly described. We used newly developed sensors allowing for simultaneous measurement of H2S and H2 or O2 and H2 within the same point to elucidate the interactions between oxygen, sulfate reducing bacteria, and H2 producing microbes. After onset of darkness and subsequent change from oxic to anoxic conditions within the uppermost ∼1 mm of the mat, H2 accumulated to concentrations of up to 40 μmol L-1 in the formerly oxic layer, but with high variability among sites and sampling dates. The immediate onset of H2 production after darkening points to fermentation as the main H2 producing process in this mat. The measured profiles indicate that a gradual disappearance of the H2 peak was mainly due to the activity of sulfate reducing bacteria that invaded the formerly oxic surface layer from below, or persisted in an inactive state in the oxic mat during illumination. The absence of significant H2 consumption in the formerly oxic mat during the first ∼30 min after onset of anoxic conditions indicated absence of active sulfate reducers in this layer during the oxic period. Addition of the methanogenesis inhibitor BES led to increase in H2, indicating that methanogens contributed to the consumption of H2. Both H2 formation and consumption seemed unaffected by the presence/absence of H2S.

Benthic anoxia, intermittent photic zone euxinia and elevated productivity during deposition of the Lower Permian, post-glacial fossiliferous black shales of the Paraná Basin, Brazil

Here, the Lower Permian, post-glacial fossiliferous Lontras black shales from the Paraná Basin (southern Brazil) are studied using integrated palynological, geochemical and petrographic methods for the first time in order to decipher the prevalent palaeoenvironmental conditions during their sedimentation. These black shales were deposited in a restricted marine environment. Inorganic geochemical data (U/Th ratios, authigenic uranium, molybdenum), organic geochemical data (total organic carbon, biomarkers) and framboid pyrite size distributions point to predominantly anoxic/euxinic bottom-water conditions. Moreover, the presence of aryl isoprenoids and maleimide biomarkers indicates that euxinia in the water column was intermittently present in the photic zone. The onset of anoxic conditions was caused by elevated productivity in the basin, which was related to deglaciation, marine transgression and the increased delivery of terrestrial nutrients. The presence of a positive organic carbon isotope excursion indicates that the black shale deposition resulted from increased productivity and the expansion of anoxic and nitrogen- and phosphate-enriched waters into the shallow photic zone. The high values of δ15N (exceeding 9‰) may be related to the deglaciation-driven sea-level rise and advection of denitrified water mass from the Panthalassic Ocean to the intracratonic Paraná Basin. Prolonged periods of sea-floor anoxia/euxinia excluded potential scavengers and bioturbators, thus enhancing the preservation of numerous fossil taxa, including fish, sponges, insects and their larval cases, and conodont apparatuses. The intermittent photic zone euxinia may also have contributed to the mass mortality of fish populations, the fossils of which are very well-preserved in these black shales.

The onset of anoxic conditions in the early Barremian of the Boreal Realm evidenced by benthic foraminifera

The Frielingen clay pit, situated approximately 20km northwest of Hannover (northern Germany) exposes sediments of Early Cretaceous age (Hauterivian, Barremian). Rhythmic alternations of dark mudstones and pale marls of late Hauterivian age (Simbirskites discofalcatus ammonite Zone) dominate the lithology. These bedding rhythms are overlain by laminated dark shales and lighter mudstones, assigned to the lowermost Barremian (Praeoxyteuthis pugio belemnite Zone). For this study 27 samples covering the Hauterivian/Barremian boundary interval were investigated with respect to their CaCO3 and total organic contents (TOC), stable isotope composition (δ13Corg, δ13CCaCO3, δ18OCaCO3) and their micropalaeontology (foraminifera). The samples were analysed in order to better understand the onset of the Barremian anoxic conditions in the Boreal Realm. The data obtained show that shifts of the environmental parameters are reflected by the mudstone-marl alternation. The dark mudstones reflect cooler surface water temperatures and a lower oxygen content, while the pale marls were deposited under warm surface water conditions. These bedding rhythms are thought to be the result of climatic variations in the Milankovitch Band (10Ka–1Ma). This study further documents that the Hauterivian/Barremian transition is characterised by a deterioration of the living conditions for benthic organisms. This is indicated by the decline of several foraminiferal genera, by decreasing generic richness and by a coeval increase in the abundances of specific taxa. The TOC content and the isotopic composition of the organic carbon support these findings. A morphogroup analysis of the benthic foraminifera indicates changes of the habitats from mainly infaunal in the late Hauterivian to more epifaunal in the early Barremian. Due to this dramatic decline of infaunal forms in the laminated, TOC-rich mudstones of the lowermost Barremian, suboxic to anoxic conditions are postulated for these sediments. The bottom water must, however, have contained some oxygen at least, as is indicated by a few epifaunal genera that became very abundant (e.g. Ammodiscus). A major regression around the Hauterivian/Barremian boundary caused a closing of the seaway to the Tethys, resulting in a significantly reduced water exchange and the establishment of anoxic conditions.

Use of an oxygen-releasing compound to aerate eutrophic reservoir water

This study examined the use of an Oxygen Release Compound (ORC®) as a slow release chemical aeration agent within eutrophic surface water systems. Bench and pilot-scale experiments involving ORC® treatment of eutrophic reservoir water were conducted to determine operational properties of ORC® and examine its performance as an aeration agent within a surface water environment. The bench-scale study involved the application of 10, 25 and 50 g ORC® doses to 1 L Erlenmeyer flasks containing various water and sediment samples. The pilot-scale study involved a scaled up simulation of full-scale reservoir systems using large fiberglass tanks, where water quality parameters in an ORC®-treated tank were compared to an untreated tank over a 4 month period. The results of these experiments indicate that the application of ORC® can result in substantial increases in dissolved oxygen and pH when applied to deoxygenated water systems. Within the pilot scale study, a 300 g/m2 dose of ORC® at the sediment water interface prevented the onset of anoxic conditions over a 4 month growing season period, releasing approximately 20% of its mass as oxygen within this time frame.

Compaction effects on CO 2 and N 2O production during drying and rewetting of soil

The effects of compaction on soil porosity and soil water relations are likely to influence substrate availability and microbial activity under fluctuating soil moisture conditions. We conducted a short laboratory incubation to investigate the effects of soil compaction on substrate availability and biogenic gas (CO 2 and N 2O) production during the drying and rewetting of a fine-loamy soil. Prior to initiating the drying and wetting treatments, CO 2 production (−10 kPa soil water content) from uncompacted soil was 2.3 times that of compacted soil and corresponded with higher concentrations of microbial biomass C (MBC) and dissolved organic C (DOC). In contrast, N 2O production was 67 times higher in compacted than uncompacted soil at field capacity. Soil aeration rather than substrate availability (e.g. NO 3 − and DOC) appeared to be the most important factor affecting N 2O production during this phase. The drying of compacted soil resulted in an initial increase in CO 2 production and a nearly two-fold higher average rate of C mineralization at maximum dryness (owing to a higher water-filled pore space [WFPS]) compared to uncompacted soil. During the drying phase, N 2O production was markedly reduced (by 93–96%) in both soils, though total N 2O production remained slightly higher in compacted than uncompacted soil. The increase in CO 2 production during the first 24 h following rewetting of dry soil was about 2.5 times higher in uncompacted soil and corresponded with a much greater release of DOC than in compacted soil. MBC appeared to be the source of the DOC released from uncompacted soil but not from compacted soil. The production of N 2O during the first 24 h following rewetting of dry soil was nearly 20 times higher in compacted than uncompacted soil. Our results suggest that N 2O production from compacted soil was primarily the result of denitrification, which was limited by substrates (especially NO 3 −) made available during drying and rewetting and occurred rapidly after the onset of anoxic conditions during the rewetting phase. In contrast, N 2O production from uncompacted soil appeared to be primarily the product of nitrification that was largely associated with an accumulation of NO 3 − following rewetting of dry soil. Irrespective of compaction, the response to drying and rewetting was greater for N 2O production than for CO 2 production.

Effects of oxygen and nitrate on nutrient release from profundal sediments of a large, oligo-mesotrophic reservoir, Lake Mathews, California

Lake Mathews is a large, oligo-mesotrophic reservoir located in Southern California. The reservoir has elevated levels of nitrate and periodically experiences hypolimnetic anoxia. Experimental sediment-water chamber incubations and reservoir water quality monitoring were conducted to evaluate how oxygen and nitrate in overlaying water affect nutrient release from profundal sediments and internal nutrient loading. In experimental incubations, under nitrate-free anoxic conditions, sediment nutrient release rates were 3.4 ± 0.8 mg-P/m2 ·d for phosphate and 2.8 ± 1.2 mg-N/m2 ·d for ammonia (average ± standard deviation; n = 6). Oxygen repressed phosphate release and greatly diminished ammonia release from sediments in experimental incubations while nitrate only repressed phosphate release. Similar nutrient release dynamics were observed in the reservoir. Nutrient release rates estimated from seasonal nutrient profiles collected from the reservoir were 3.4 mg-P/m2 ·d for phosphate and 2.5 mg-N/m2 ·d for ammonia. Ammonia accumulation in the hypolimnion commenced with the onset of anoxic conditions, but phosphate accumulation did not start until nitrate disappeared from bottom waters approximately 6 weeks later. The time lag decreased total internal phosphorus loading by approximately 25% relative to hypothetical nitrate-free conditions. Laboratory and field data show that both oxygen and nitrate repress sediment phosphate release, likely via the maintenance of an oxidized surficial sediment layer that retains phosphate in iron-oxide complexes. However, only oxygen and not nitrate was effective in decreasing sediment ammonia release, likely by enhancing biological nitrification and assimilation in surficial sediments under oxic conditions. A number of in-lake management strategies have been developed to inhibit internal nutrient loading including calcium nitrate addition, aluminum sulfate addition, and oxygenation. In our view, the deliberate addition of nitrate to lakes and reservoirs poses several risks that must be carefully considered when evaluating strategies to control sediment phosphorus release.

Inhibition of ammonia release from anoxic profundal sediments in lakes using hypolimnetic oxygenation

Ammonia is an important compound in freshwater ecosystems. It can stimulate phytoplankton growth, exhibit toxicity to aquatic biota, and exert an oxygen demand in surface waters. In productive lakes, ammonia commonly accumulates in bottom waters in conjunction with the onset of anoxic conditions, thereby degrading lake water quality. Sediment cores from 12 lakes and reservoirs were incubated in experimental chambers to examine how dissolved oxygen levels impact sediment release of ammonia. Sediments from all sites released ammonia under anoxic conditions. Release rates for oligo/mesotrophic, meso/eutrophic, and eutrophic/hypereutrophic sites were <5, 5–10, and >15 mg-N m −2 day −1, respectively. Under oxic conditions, ammonia release was negligible in sediments from oligotrophic and mesotrophic sites, and generally lower or reversed in sediments from eutrophic sites. Ammonia release under anoxic conditions is the result of ammonia build up in sediments due to a loss of biological nitrification and a decrease in ammonia assimilation by anaerobic microorganisms. Lake oxygenation, the dissolution of pure oxygen gas into surface waters, can improve water quality in the bottom of thermally stratified lakes, and potentially in shallow lakes, by eliminating sediment ammonia release through the maintenance of a well-oxygenated sediment–water interface.

Palaeoenvironments of the Permian–Triassic transition sections in Kashmir, India

Detailed measurements of two Permian–Triassic boundary sections at Guryul Ravine and Pahlgam in Kashmir indicate continuous sedimentation across the boundary, though with a marked change from shallow storm-influenced shelf to deeper shelf below wave base. Successive shell beds through the 2.5-m-thick transitional latest Permian section at Guryul formed as in situ accumulations with little sign of either erosional breaks or condensation. Faunal change takes place gradually in the late Permian transition beds, as elsewhere in the Tethyan sections. There is no major event layer at the Permian–Triassic boundary, and little evidence for dysaerobic conditions. Both the Kashmir and East Greenland Permian–Triassic sections have sedimentation rates that are orders of magnitude greater than at the condensed Meishan reference section. They show that the end-Permian extinction was associated with transgression but not with immediate onset of anoxic conditions. Macrofaunal extinction seems associated with reduction in primary production, which would also explain the dwarfism of fossils found in all Permian–Triassic sections studied so far.

Fe(III) Oxide Reactivity Toward Biological versus Chemical Reduction

Initial rates of biological (Shewanella putrefaciens strain CN32, pH 6.8) and chemical (ascorbate, pH 3.0) reduction of synthetic Fe(III) oxides with a broad range of crystallinity and specific surface area were examined to assess how variations in these properties are likely to influence the kinetics of bacterial Fe(III) oxide reduction in heterogeneous natural Fe(III) oxide assemblages. The results indicate that bacterial Fe(III) oxide reduction does not respond strongly to oxide crystal thermodynamic properties (ΔGf) which exert a significant impact on the kinetics of abiotic reductive dissolution. These findings suggest that oxide mineral heterogeneity in natural soils and sediments is likely to affect initial rates of bacterial reduction (e.g. during the early stages of anaerobic metabolism following the onset of anoxic conditions) mainly via an influence on reactive surface site density and that inferences regarding the competitiveness of bacterial Fe(III) oxide reduction as a pathway for organic matter oxidation in anoxic environments cannot be based on assumed thermodynamic properties of the dominant oxide phase(s) in the soil or sediment.

Impact of late glacial climate variations on stratification and trophic state of the meromictic lake Längsee (Austria): validation of a conceptual model by multi proxy studies

Selected pigments, diatoms and diatom-inferred phosphorus (Di-TP) concentrations of a late glacial sediment core section of the meromictic Längsee, Austria, were compared with tephra- and varve-dated pollen stratigraphic and geochemical results. A conceptual model was adopted for Längsee and evaluated using multi proxy data. During the unforested late Pleniglacial, a holomictic lake stage with low primary productivity prevailed. Subsequent to the Lateglacial Betula expansion, at about 14,300 cal. y BP, okenone and isorenieratene, pigments from purple and green sulphur bacteria, indicate the onset of anoxic conditions in the hypolimnion. The formation of laminae coincides with this anoxic, meromictic period with high, though fluctuating, amounts of okenone that persisted throughout the Lateglacial interstadial. The occurrence of unlaminated sediment sections of allochthonous origin, and concurrent low concentrations of okenone, were related to cool and wet climate fluctuations during this period, probably coupled with a complete mixing of the water column. Two of these oscillations of the Lateglacial interstadial have been correlated tentatively with the Aegelsee and Gerzensee oscillations in the Alps. The latter climate fluctuation divides a period of enhanced anoxia and primary productivity, correlated with the Alleröd chronozone. Continental climate conditions were assumed to be the main driving forces for meromictic stability during Alleröd times. In addition, calcite dissolution due to severe hypolimnetic anoxia, appear to have supported meromictic stability. Increased pigment concentrations, which are in contrast to low diatom-inferred total phosphorus (Di- TP), indicate the formation of a productive metalimnion during this period, probably due to a clear-water phase (low catchment erosion), increased temperatures, and a steep gradient between the phosphorus enriched hypolimnion and the oligotrophic epilimnion. Meltwater impacts from an extended snow-cover and a summer temperature decline, together with climate instability, are assumed to be the main reasons for the lowering of the anoxic level during the following, climatically heterogeneous, Younger Dryas. Meromictic stability was re-established with the termination of the Younger Dryas. The sequence of key pigments, Di-TP, calcite, siliciclastics, and organic carbon, seem to be in accordance with the assumptions of the conceptual model.

Thermal, mixing, and oxygen regimes of the Salton Sea, California, 1997–1999

The Salton Sea is a shallow (mean depth = 8 m; maximum depth = 15 m), saline (41–45 g l−1), intermittently mixing, 57 km long, 980 km2 lake located in the arid southwestern United States. The Sea is a wind driven system, with predominant winds paralleling the long axis of the lake, being strongest in spring and weakest in summer and fall. The Sea mixed daily or nearly daily between September and January. During this cooling period, moderate to high levels of dissolved oxygen (3–11 mg l−1) were found throughout the water column. Mean water column temperature ranged from a minimum of 13–14 °C in early January to a maximum of 30–34 °C in July–September. During most of this warming period, the Sea was thermally stratified but subject to periodic wind driven mixing events. Winds were stronger in spring 1998 than in 1997 or 1999, causing more rapid heating of the lake that year and also delaying onset of anoxic conditions in bottom waters. During summer months, mid-lake surface waters were sometimes supersatured with oxygen, and bottom waters were hypoxic or anoxic with sulfide concentrations > 5 mg l−1. Oxic conditions (> 1 mg O2 l−1) often extended a few meters deeper nearshore than they did well offshore as a consequence of greater mixing nearshore. Mixing events in late summer deoxygenated the entire water column for a period of days. Consumption of oxygen by sulfide oxidation likely was the principal mechanism for these deoxygenation events. Sulfide concentrations in surface waters were 0.5–1 mg l−1 approximately 3 days after one mixing event in mid-August 1999. These mixing events were associated with population crashes of phytoplankters and zooplankters and with large fish kills. In the southern basin, freshwater inflows tended to move out over the surface of the Sea mixing with saline lake water as a function of wind conditions. Salinity gradients often contributed more to water column stability than did thermal gradients in the southeasternmost portion of the lake.

Benthic foraminiferal successions across Late Quaternary Mediterranean sapropels

Benthic foraminiferal records across Late Quaternary Mediterranean sapropels S6, S5 and S1 are reviewed and re-considered in the light of recent advances in foraminiferal ecology. It is suggested that the main factor controlling the foraminiferal successions that occur immediately before and after sapropel deposition is the amount of time involved in the onset of anoxic conditions and in the re-oxygenation of the benthic environment. Faunas dominated by deep infaunal taxa such as Globobulimina and Chilostomella reflect a very gradual decrease or increase of bottom water oxygenation. Post-sapropel faunas dominated by small biconvex, trochospiral taxa, which are inferred to have a much more opportunistic life strategy, are typical of a very rapid re-oxygenation of the benthic environment after sapropel deposition. The time involved in re-oxygenation depends upon the mechanism causing sapropel formation. The faunal succession found above sapropel S5 suggests a basin with a strong density gradient in the upper part of the water column during sapropel deposition, followed by a rapid turnover of the following column and quick re-oxygenation of the benthic environment. In contrast, the faunal succession above sapropel S6 suggests a water column with a more gradual density gradient deeper in the basin, a progressive deepening of the halocline, and a very slow increase of bottom water oxygenation.

Early diagenetic production and sediment-water exchange of fluorescent dissolved organic matter in the coastal environment

Fluorescence at wavelengths characteristic of humic substances (excitation 350 nm, emission 450 nm) have been used in this study to approximate concentrations of fluorescent dissolved organic material (FDOM). In situ regulated and unregulated benthic chambers, sediment cores, and laboratory tank incubations were used to study early diagenesis of FDOM in coastal marine sediments of the Gullmar Fjord, western Sweden. In the regulated in situ chambers, pH and oxygen were kept at relatively stable levels, while in the unregulated in situ chambers, pH and oxygen were left to decrease as a result of biological activity. FDOM porewater distributions and correlation between FDOM and parameters indicating mineralization showed that FDOM was formed in the sediment and should flux across the sediment-water interface. A substantial flux of FDOM was also observed during winter and spring conditions and during anoxic conditions fall. However, no flux was observed during oxic conditions fall. Modeling indicated that oxygen penetration depth was deeper during winter than during fall, i.e., the oxygen penetration depth increased during fall towards winter values. We suggest that as FeOOH was formed when oxygen penetration depths increased, FROM was sorbed to newly formed FeOOH, inhibiting FDOM flux over the sediment-water interface. In addition, at onset of anoxic conditions in the sediment surface layer in fall incubations, FDOM flux from sediment to overlying water increased substantially. Increases in anoxic FDOM fluxes were accompanied by increases in Fe and phosphate fluxes. We suggest that reductively dissolved FeOOH released sorbed FDOM. FDOM released from FeOOH by anoxic conditions was not resorbed when oxic conditions were resumed. This could be an effect of higher pH in overlying water as compared with porewater, inhibiting FeOOH sorption of FDOM.

A Neoproterozoic multi-phase rift sequence: the Grampian and Appin groups of the southwestern Monadhliath Mountains of Scotland

The Grampian and Appin groups of the southwestern Monadhliath Mountains form the earliest known syn-rift sequences of the Scottish central Highlands. They were likely to have formed in an intracontinental setting and represent deposition of mixed clastic and carbonate shallow and deep marine strata. The Grampian Group of the southern Monadhliath Mountains was deposited during a period of initial basin rifting (NW–SE extension) followed by a phase of thermal subsidence. Syn-rift sediments comprise a 2.5–6 km thick turbidite system. Thermal subsidence brought about the basinward progradation of shallow marine shelf sediments resulting in the infilling of pre-existing basin topography. The overlying Appin Group commenced with deposition of a shallow marine sequence alternating between nearshore tidal sand and offshore mud deposition. This formed in response to renewed rifting and concomitant subsidence. Accelerated rifting resulted in localized footwall uplift and erosion while sedimentation continued in the hanging-wall areas. Resultant subsidence, perhaps partly thermally driven, caused gradual basin widening and produced an onlapping marine sequence. There followed a period of progressive clastic deprivation when carbonates were precipitated, and at the onset of anoxic conditions, deposition of organic muds. The fundamental structural elements responsible for the formation of the Grampian and Appin group basins were also influential in the orogenic evolution of the basin-fill. Half-graben fills were deformed to produce regionally extensive folds such as the Stob Ban–Craig a’ Chail Synform.

Anoxia in the Northern Adriatic Sea: rapid death, slow recovery

Abstract The Northern Adriatic Sea is characterized by the full range of features associated with sensitive marine ecosystems. In the Gulf of Trieste, mortalities due to anoxia have been reported in 1974, 1983 and 1988. The anoxic event in September 1983 led to mass mortality in a highly developed and wide-ranging community in the Gulf. This destroyed over 50% of the epifaunal biomass in only two days, and over 90% within four days. The increased frequency, severity, and range of recent oxygen deficiencies suggest that, in addition to meteorological and hydrographic factors, anthropogenic influences are a contributing or enhancing factor in the onset of anoxic conditions and associated mass mortalities. These anthropogenic influences may be subsumed under the heading eutrophication. If eutrophication only slightly increases the rate of oxygen decrease below the thermocline, it may advance the point at which critical oxygen levels are attained by several days. Since only a few days of anoxic conditions are required to destroy this benthic community (with an equally short time required for reoxygenation) the degree of eutrophication may determine whether an ecological catastrophe occurs or not. The recolonization process between 1984 and 1987 has been slow: the total biomass and the relative contribution of the designating species have remained distinctly below previously recorded values. Additional anthropogenic influences (intensive dredging) may further impede recolonization. A further, small-scale mortality was registered in 1988.

Production and Loss of Nitric Oxide from Denitrification in Anaerobic Brookston Clay

Nitric oxide, nitrous oxide, and nitrite ion production was measured in a Brookston clay column undergoing anaerobic denitrification. A flow system method was used whereby argon carrier gas continuously stripped soil gases from the column, allowing steady-state rates to be obtained. Over several days the temporal change in rates of these gases and NO(2) followed a pattern of increase and decay which may be expected of a reaction proceeding by several consecutive steps. The method permits observation of the relatively large net production rate of NO, which is normally not observed in static systems based on head space analysis of gaseous denitrification products. In the first several hours after the onset of anoxic conditions, the net rate of NO production, f(NO), increased sharply to a maximum ( approximately 1 x 10 mol of N/g of soil per min), paralleling the rapid increase in NO(2) level, and then followed a more gradual decline extending over approximately 45 h. A similar but less pronounced pattern was observed for N(2)O, with net rates of production being considerably less than for NO. The ratio [NO-N]/[N(2)O-N] decreased with time from approximately 2.5 at 6 h to approximately 2.0 at 45 h. Estimated rates of N(2) production appeared to be initially high, decreased rapidly within a few hours, and then gradually increased with time after the establishment of anaerobic conditions.